1. Technical Field
The present invention relates to coatings for plastic materials and methods for manufacturing coated plastic materials.
2. Related Art
Plastic materials such as acrylonitrile-butadiene-styrene (ABS), polyethylene (PE), polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) or polyethylene terephthalate (PET) have been extensively developed, and are used in numerous applications such as automobile parts and accessories, containers, household appliances, and other commercial and consumer items.
However, plastic materials have the following disadvantages in general. Firstly, after injection molding plastic material to form a plastic product, the plastic product is prone to shrinking and deformation when it cools to room temperature. The amount of shrinkage is difficult to control. Secondly, a linear thermal expansion coefficient of plastic materials is large. Thirdly, a shrinkage temperature of plastic materials is relatively low. For example, the shrinkage temperature of PMMA is 95° C. Finally, plastic materials have water absorption propensity. For example, the water absorption rate of PMMA is 2 percent at 40° C.
To circumvent the above disadvantages, organic coatings are employed to protect plastic materials from degradation when exposed to atmospheric weathering conditions such as sunlight, moisture, heat and cold. In addition, the organic coatings can serve as decoration. To achieve longer lasting and more durable properties, it is necessary for the organic coatings to be tightly adhered to the surface of the plastic material.
Referring to FIG. 4, a conventional organic coating 12 is directly formed on a plastic substrate 14 to form a coated substrate 10. Due to the limitation of the glass transition temperature (Tg) of the plastic substrate 14, the temperature of the plastic substrate 14 must be maintained at a lower level when coating. Preferably, such temperature is not higher than two-thirds of the Tg of the plastic. During the coating process, coating materials are deposited on the plastic substrate 14 by way of a phase change process from gas to solid, and heat is thereby generated. Therefore a temperature at an interface between the organic coating 12 and the plastic substrate 14 is increased. Accordingly, an unduly large amount of water is absorbed by the plastic substrate 14, and water exists at the interface between the organic coating 12 and the plastic substrate 14. The presence of the water reduces the binding energy between the organic coating 12 and the plastic substrate 14.
Thus, it is difficult to achieve good adhesion of the organic coating 12 to the plastic substrate 14. In particular, the bond between the organic coating 12 and the plastic substrate 14 is liable to weaken after aging or environmental exposure. After a time, the organic coating 12 is liable to separate from the plastic substrate 14, resulting in cracking and chipping. FIG. 5 shows a microscopic view of an impaired interface between the organic coating 12 and the plastic substrate 14.
Therefore, a heretofore-unaddressed need exists in the industry to overcome the aforementioned deficiencies and inadequacies.